Re: [Qemu-devel] [PATCH for 2.6 1/3] backup: Use Bitmap to replace "s->bitmap"

[Fam Zheng <famz@redhat.com>]

On Mon, 11/23 17:24, Wen Congyang wrote:
> On 11/23/2015 05:19 PM, Fam Zheng wrote:
> > On Mon, 11/23 17:01, Wen Congyang wrote:
> >> On 11/20/2015 05:59 PM, Fam Zheng wrote:
> >>> "s->bitmap" tracks done sectors, we only check bit states without using any
> >>> iterator which HBitmap is good for. Switch to "Bitmap" which is simpler and
> >>> more memory efficient.
> >>>
> >>> Meanwhile, rename it to done_bitmap, to reflect the intention.
> >>>
> >>> Signed-off-by: Fam Zheng <famz@redhat.com>
> >>> ---
> >>>  block/backup.c | 11 ++++++-----
> >>>  1 file changed, 6 insertions(+), 5 deletions(-)
> >>>
> >>> diff --git a/block/backup.c b/block/backup.c
> >>> index 3b39119..d408f98 100644
> >>> --- a/block/backup.c
> >>> +++ b/block/backup.c
> >>> @@ -22,6 +22,7 @@
> >>>  #include "qapi/qmp/qerror.h"
> >>>  #include "qemu/ratelimit.h"
> >>>  #include "sysemu/block-backend.h"
> >>> +#include "qemu/bitmap.h"
> >>>  
> >>>  #define BACKUP_CLUSTER_BITS 16
> >>>  #define BACKUP_CLUSTER_SIZE (1 << BACKUP_CLUSTER_BITS)
> >>> @@ -47,7 +48,7 @@ typedef struct BackupBlockJob {
> >>>      BlockdevOnError on_target_error;
> >>>      CoRwlock flush_rwlock;
> >>>      uint64_t sectors_read;
> >>> -    HBitmap *bitmap;
> >>> +    unsigned long *done_bitmap;
> >>>      QLIST_HEAD(, CowRequest) inflight_reqs;
> >>>  } BackupBlockJob;
> >>>  
> >>> @@ -113,7 +114,7 @@ static int coroutine_fn backup_do_cow(BlockDriverState *bs,
> >>>      cow_request_begin(&cow_request, job, start, end);
> >>>  
> >>>      for (; start < end; start++) {
> >>> -        if (hbitmap_get(job->bitmap, start)) {
> >>> +        if (test_bit(start, job->done_bitmap)) {
> >>>              trace_backup_do_cow_skip(job, start);
> >>>              continue; /* already copied */
> >>>          }
> >>> @@ -164,7 +165,7 @@ static int coroutine_fn backup_do_cow(BlockDriverState *bs,
> >>>              goto out;
> >>>          }
> >>>  
> >>> -        hbitmap_set(job->bitmap, start, 1);
> >>> +        bitmap_set(job->done_bitmap, start, 1);
> >>
> >> You can use set_bit() here.
> > 
> > Why? I think bitmap_set is a better match with bitmap_new below.
> 
> set_bit() is quicker than bitmap_set() if you only set one bit.
> 

How much quicker is it? This doesn't sound convincing enough for me to lose the
readability.

Fam